The present invention relates to shim members used to space apart stacked porous substrates during a manufacturing process. A particular example of the present invention relates to metallic or carbon annular shim members used to space apart stacked annular composite material preforms especially during a densification process, such as chemical vapor infiltration (CVI).
The composite material preforms may particularly be annular preforms for making brake disks or other friction members.
An apparatus for densifying annular preforms to make brake disks and the like is disclosed in, for example, U.S. patent application Ser. No. 10/468,031 filed on Aug. 14, 2003; a representation thereof is illustrated in FIG. 1.
FIG. 1 is a highly diagrammatic illustration of a process chamber having an enclosure 10 therein containing a load of annular preforms or substrates 20 made from carbon fiber. The load is in the form of a stack of substrates having their respective central passages generally in vertical alignment. The stack may be made up of a plurality of superposed stack sections separated by one or more intermediate support plates 12.
The stacked substrates are separated from one another by means of spacers 30. As shown in FIG. 2, the spacers 30 may be disposed radially, and the number of them may vary. They provide gaps 22 of substantially constant height throughout the entire stack between adjacent substrates, while allowing the inside volume 24 of the stack, as constituted by the generally aligned central passages of the substrates, to communicate with the outer volume 26 situated outside the stack and inside the enclosure 10.
In the example of FIG. 1, the enclosure 10 contains a single stack of substrates. In a variant, a plurality of stacks of substrates may be disposed side by side in the same enclosure.
The enclosure 10 is heated by means of a susceptor 14, e.g. made of graphite, which serves to define the enclosure 10 and which is inductively coupled with an induction coil 16 situated outside a casing 17 surrounding the susceptor. Other methods of heating may be used, for example resistive heating (the Joule effect).
A gas containing one or more precursors of carbon, typically hydrocarbon gases such as methane and/or propane, is admitted into the enclosure 10. In the example shown, admission takes place through the bottom 10a of the enclosure. The gas passes through a preheater zone 18 formed by one or more pierced plates disposed one above another in the bottom portion of the enclosure, beneath the plate 11 supporting the stack of substrates. The gas heated by the preheater plates (which are raised to the temperature that exists inside the enclosure) flows freely into the enclosure, passing simultaneously into the inside volume 24, into the outer volume 26, and into the gaps 22. The residual gas is extracted from the enclosure by suction through an outlet formed in the cover 10b. 
Spacers 30 are individually placed block members, most usually made from alumina. However, once formed, the alumina block members are very fragile, and losses from breakage are very high. In fact, in normal usage, the conventional alumina blocks frequently last not more than 2 or 3 densification cycles. This naturally raises manufacturing costs, as the alumina blocks must be replaced.
Moreover, the proper manual placement of individual alumina block members between each preform layer is extremely time-consuming. Six such block members are shown in FIG. 2 by way of illustrative example, and in actual practice as many as twelve blocks are used. The time burden is exacerbated by the extraordinary care needed to handle the fragile blocks without breakage. In general, a full densification process comprising seven trays of preforms (each with twelve to fourteen preform stacks) can take as long as one or two working days to set up according to the conventional method.
Another problem related to the use of individual spacer members 30 is that they tend to cause deformations (literally, dents) in the preforms caused by the weight of preforms (and spacers) stacked thereabove. As can be appreciated from FIG. 2, there are large unsupported areas of the preform circumferentially between the spacer members 30. Because the preform material is generally pliable, and because the alumina constituting spacer members 30 does not deform, indentations occur in the surface of the preforms in locations corresponding to the spacer members 30. These deformations, however slight, must be machined away in an extra finishing step so as to obtain a desirably planar surface usable for friction applications. As a result, the thickness of each preform is thicker than is needed for a final product, in anticipation of the deformations that occur in the known process and of the final machining step to remove those deformations. The machined-away material represents economic waste.